In no particular order...
Roll back the weirder "balance" changes that made no sense. There's better ways than adding gimmicks or completely changing the role of a perfectly good weapon. (Looking at you Claidheamh, Bison, Shortstop, and Pocket Pistol)
Give Heavy a reason to use something other than the Sandvich, especially the blinking shotgun. Lunchbox, buff the other consumables, a primary replacement that buffs other stats, whatever.
Give Pyro an actual dedicated definable role that is the same both within casual and comp. A kinda flanking, complacently comboing, really only uber-negating jack-shite of all trades helps no one.
Implement balance changes that reflect weapon appearances but also make them viable (Maybe I'm OCD, but I hate it when a weapon's shape means squat diddly to its stats). Stock grenade launcher has 6 shots but lower damage to cut down spam complaints. Back Scatter has 8 shots to give people an actual reason to use it. Not really balance, but fix shotgun rounds going out the gun not where the ejector is. You get the picture.
Add tutorial mode for every class. Lets noobs learn how to play before they take a match, gives pros a place to practice the advanced techniques, and is perfect fodder to give more backstory and character to everyone. Just have the tutorials be phase-selectable sorta "single player campaign levels" so to speak.